#### Answer

The percent by mass of sulfuric acid in this solution is: $44.0 \%$

#### Work Step by Step

1. Calculate the molar mass of the sulfuric acid:
H: 1.0 * 2 = 2.0
S: 32.1 * 1 = 32.1
O: 16.0 * 4 = 64.0
2.0 + 32.1 + 64.0 = 98.1 g/mol.
2. Now, convert the concentrations into g/L.
$H_2SO_4: \frac{6.00 mol}{1 L} \times \frac{98.1g}{1 mol} = \frac{588.6 g}{1 L}$
$Solution: \frac{1.338g}{1 ml} \times \frac{1000ml}{1L} = \frac{1338g}{1L}$
3. Find the percent of mass, considering the same volume:
$Percent = \frac{588.6g}{1338g} \times 100\% = 44.0\%$